This question seeks answers with one of the following as supporting evidence: International standards, other standards widely recognized in multiple areas of the United States, or sound building and engineering principles.
Note: If there is a preferred material for the vent line in this application, I would also greatly appreciate a recommendation. As a theoretical example: "Use PVC instead of galvanized steel because PVC will help to insulate against the heat of the chimney."
Why I ask this question:
As part of a basement remodel, I need to upgrade an existing plumbing line and add venting through the roof for the fixtures it serves.
There is a roughly 4" gap between a brick chimney (which provides the exhaust for my furnace and hot water tank) and one of the walls all the way to the attic.
This leaves enough room for a 2" waste vent line and a roughly 2" air gap if I were to run the vent line by the wall next to the chimney. Then, I do not have to drill any new holes in floors or ceilings to add the vent line. However, I realize that the methane gas from waste is combustible and running something combustible that close to a hot chimney could make it a bad idea.
Best Answer
The ultimate standard is going to be your local building department and inspector, but the International Plumbing Code only specifically prohibits plumbing in elevator shafts or in elevator equipment rooms (301.6). The only requirements specific to location of venting deal with size, distance, and type of connection to the system that they are venting. Everything else is deferred to the International Mechanical Code.
For chimneys that are used only for venting gas-fired appliances, the IMC defers to the International Fuel Gas Code. The relevant portion (501.15.4) of that code states only that there be airspace clearance to combustibles per the International Building Code. Note that there is a specific exemption for masonry chimneys with a low clearance liner.
This gets you back to what it appears that you already know from your question (R1003.18) "Any portion of a masonry chimney located in the interior of the building or within the exterior wall of the building shall have a minimum air space clearance to combustibles of 2 inches (51 mm)." However, again this is exempted if a low clearance flue liner is in place:
PVC will ignite at 435°-557°, roughly the same temperature range wood will (although it will be a molten puddle at that point). So, unless your chimney liner is rated for contact with combustibles you would need galvanized or some other form of non-combustible pipe. If you don't know how the chimney is rated, I would take the presence of the existing air gap to indicate that it is not contact rated.
That said, if you can maintain the 2" minimum to a PVC pipe and the local code authority will sign off on it, I'd do it. Oversimplified, a 90% efficient gas furnace would basically dump only 10% of it's BTU output into the flue gas. I don't ever recall finding a brick chimney that was even warm to the touch on the outside, except right next to the inlet from the furnace.